The present invention relates to projectiles containing a flux compression generator (FCG) for producing a high current that acts to produce a metal mass in a manner to project that mass as a jet to penetrate a target.
Flux compression generators are already known in the art. An example thereof is disclosed in U.S. Pat. No. 4,370,576, Foster, Jr., issued on Jan. 25, 1983, and the entirety of which is incorporated herein by reference.
It is known that extremely high magnetic fields can be obtained using high explosives as an energy source in devices known as flux compression generators. In such a generator, an explosive detonation compresses an established low-level magnetic field into a very high density field, with an associated high electrical current flow. Typically, a low-level magnetic field is established within a confined space or cavity and acted upon by the force of explosive detonation to collapse that space to a relatively small volume in which the magnetic field is trapped and compressed. Since the trapped magnetic field exerts magnetic pressure, the explosive does work against that pressure and in the process transfers its chemical energy into electrical energy within the FCG electrical circuit to include the energy stored within the compressed magnetic field. The FCG principles apply to various geometries where the size of the space, or cavity, is reduced. To date, mostly cylindrical geometries have been explored.
There are two types of cylindrical FCGs, namely, coaxial and helical.
A coaxial generator consists of a central cavity containing a centrally located high explosive filled cylindrical shell acting as a conducting armature, a cavity between the armature and an outer metallic shell that acts as a conducting stator, and conducting end caps to complete the electrical circuit and provide confinement of the compressed magnetic field. One example of a coaxial generator that can be employed in devices according to the invention is disclosed in: J. H. Goforth, et al, “The Ranchero Explosive Pulsed Power System,” 11th IEEE International Pulsed Power Conference, Hyatt Regency, Baltimore Md., Jun. 29-Jul. 2, 1997.
A helical generator consists of a similar armature, a stator formed from windings of wires, a cavity between the armature and stator, and end caps. Generally, an electrical load, in the form of a relatively small cavity encased in conducting metals, is attached to the output end of the FCG. One example of a helical generator that can be employed in devices according to the invention is disclosed in: A. Neuber, A. Young, M. Elsayed, J. Dickens, M. Giesselmann, M. Kristiansen, “Compact High Power Microwave Generation,” Proceedings of the Army Science Conjfrence (26th), Orlando, Fla., 1-4 Dec. 2008.
In addition, an internal arrangement within the device is structured so that an electrical “seed” current can be fed to the metal wire conductors forming the circuit of the stator, armature, end caps, and electrical load that define the cavities of the FCG and the load. The flow of current in the conductors around these cavities establishes a “seed” magnetic field within the cavities. The cavities represent inductances while the conductors have electrical resistance. In operation, upon detonation, the armature expands radially and collides with the stator. During that process, flux compression takes place because the FCG cavity width is reduced to nearly zero. To first order, the FCG output current results from the starting inductances of both cavities relative to the final inductance of the system after magnetic compression. When the FCG is completely collapsed, current gain is the ratio of the initial cavity inductance to the final inductance represented by the load.
An advantage of the helical generator with its wire wound stator is that a much higher initial inductance can be obtained per unit length, but at the expense of added complexity. In contrast, the coaxial generator has a simpler construction, but with a considerably lower initial inductance. Both generators can have electrical breakdown (arcing) since the current and voltages rise during compression unless care is taken to use insulating gas in the cavities. The helical generator can also break down if the voltage between wires rises above a threshold limit related to the insulation used between windings. Further, because of Joule heating due to resistance, the wires can only carry a limited amount of current without reaching their melting temperature. For well-designed generators of similar length, typical current gains are 10 to 12 for the coaxial types, and above 2000 for a helical wound generator. Often, coaxial generators are used with much higher seed current to get high output current since premature electrical breakdown and wire melting are not issues.
When initiation of the high explosive (HE) is started at one end of the HE column, i.e. along the length of the generator, the detonation wave travels from that end to the opposite end of the column, referred to as the output end. Armature radial motion first occurs at the initiation end with a progressive expansion from the initiation end to the output end. This sequential motion results in an armature expansion that has a conical profile with the cone becoming progressively larger until successive elements strike the stator. Thus, the armature first strikes the stator at the initiation end and subsequently strikes the stator at progressive locations until impact with the entire stator is complete at the output end. As the armature progressively fills the cavity, magnetic compression progressively takes place. The progression gives rise to a near exponential increase in current to a peak value that occurs near to total cavity collapse where the system inductance has a minimum value. Thus, for the helical generator, initial winding sections are subject to relatively low voltages and temperatures while sections toward the output end approach or exceed the voltage and temperature limits. Internal voltages, electrical breakdown, and wire melting have limited the ability to develop more efficient flux compression generators. In addition, explosive initiation techniques and quality control of fabricated parts including the end caps, stators, and armatures have a major influence on the ability to improve current outputs of FCGs.
Work with explosively driven flux compression in the United States dates back to C. M. Fowler's work published in 1960: C. M. Fowler, W. B. Garn, and R. S. Caird, “Production of Very High Magnetic Fields by Implosion,” Journal of Applied Physics, 31(3), 1960, pp. 588-594.
Since then, both coaxial and helical generators have been designed, built, and tested. The most notable groups examining helically wound generators include Los Alamos National Laboratory in Los Alamos, N. Mex., as disclosed in: C. M. Fowler and L. L. Altgilbers, “Magnetic Flux Compression Generators: a Tutorial and Survey,” Journal of Electromagnetic Phenomenon, 3(11), 2003, pp. 305-357, the Kurchatov Institute of Atomic Energy in Moscow, S. Kassel, “Pulsed-Power Research and Development in the USSR,” R-2212-ARPA, May 1978, and Texas Tech University in Lubbock, Tex., A. Neuber, et al, supra.
Notable patents pertaining to explosively driven flux compression devices with helically wound generators include U.S. Pat. No. 4,370,576, J. S. Foster and J. R Wilson, U.S. Pat. No. 3,356,869, J. L. Hilton and M. J. Morley, and U.S. Pat. No. 5,059,839M. F. Rose et. al, all of which are incorporated herein by reference.
U.S. Pat. No. 4,370,576 details the operation of helically wound flux compression generators. J. L. Hilton's patent claims the use of complex winding patterns to enhance electrical efficiency for flux compression devices. M. F. Rose patent outlines a flux compression/transformer system for use with high impedance loads.
The cited developments, while exploratory in nature, have not resulted in efficient FCG designs. Mainly, the threshold limits have been low while some FCG's have been relatively large and heavy with low current gains. Further, applications to weaponry have not been forthcoming because of FCG low-output, large size, awkward packaging into warhead compartments within projectiles or missiles, and requirement for external power sources to produce seed current. In addition, for weaponry that deliver lethal kinetic energy, use of FCG's with dynamic loads to produce kinetic energy penetrators and multiple kinetic energy effects has not been investigated.